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The toxin, microcystin, is produced by the algae microcystis. The Guardian explains that it's:

a harmful blue-green algae; it causes skin rashes and may result in vomiting and liver damage if ingested. It has been known to kill dogs and other animals and boiling the water does not fix the problem; it only concentrates the toxin.

"We're right back to where we were in the '70s," Jeff Reutter, head of the Ohio Sea Grant research lab told the Associated Press*. Four decades ago, the Great Lakes were so polluted that Cleveland's Cuyahoga River "oozed rather than flowed." The lakes' sorry state inspired Congress to pass the Clean Water Act; state and local officials are now in talks to see what kinds of preventative measures can be taken to halt the environmental backsliding currently going on.

The algae bloom was likely fed by high levels of phosphorus pollution in Lake Erie. Phosphorus comes from fertilizers and sewage treatment plants, and the levels of phosphorus in the lake have been increasing steadily over the past few decades.

This particular algal bloom wasn’t even expected to peak until September, according to the AP. It’s still relatively small, but winds pushed it directly into the path of Toledo’s water intake, contaminating the supply.

Toledo’s mayor has said that the water ban enacted over the weekend will remain in effect today, at least for a few more hours. Tests by the U.S. EPA and the Ohio EPA showed that the toxin had returned to safe levels, but Mayor D. Michael Collins said in an early morning press conference that the tests remained “too close for comfort,” according to the Toledo Blade.

*This article has been corrected to reflect that Jeff Reutter spoke with the Associated Press, not NPR